<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title></title>
  </head>
  <body>
    <script>
      const arr = [1, [2, [3, [4, 5]]], 6];
      console.log(arr);
      const res1 = arr.flat(Infinity);
      console.log(res1);
      const res2 = JSON.stringify(arr).replace(/\[|\]/g, "").split(",");
      console.log(res2);
      const res3 = [];
      const fn = (arr) => {
        for (let i = 0; i < arr.length; i++) {
          if (Array.isArray(arr[i])) {
            fn(arr[i]);
          } else {
            res.push(arr[i]);
          }
        }
      };
      fn(arr);
      console.log(res3);
    </script>
  </body>
</html>
